Self-ordering of anodic porous alumina formed in organic acid electrolytes

New self-ordering porous alumina films were fabricated in organic acid electrolytes. Highly ordered cell arrangements of porous alumina films were realized in malonic acid at 120 V and tartaric acid at 195 V having 300 nm and 500 nm pore intervals, respectively. Self-organization was achieved at the maximum voltage required to induce high-current-density anodization while preventing burning, i.e., an extremely high-current flow concentrated at local points. The cells of the film grown at a high field must be pressed against each other, so that the self-ordering proceeds with the porous layer growth. When the self-ordering of cell arrangement proceeds, the cells became smaller. To improve the regularity of the cell configuration, a low electrolyte temperature and a relatively high electrolyte concentration were effective for maintaining a high-current-density to prevent burning. Surface flatness was an essential factor for self-ordering, however, the surface oxide film produced by electropolishing an aluminum substrate prevented quick pore growth in the organic acids having a low dissociation constant. It is confirmed that electropolishing followed by alkaline treatment was most appropriate as the pre-treatment in preparing flat surfaces.     

Porous anodic alumina formed by anodization of aluminum alloy (AA1050) and high purity aluminum

A two-step anodization process performed at 0 °C was used to prepare highly ordered porous anodic alumina on the AA1050 alloy and high purity aluminum foil. The anodizing of both substrates was carried out in 0.3 M sulfuric acid and 0.3 M oxalic acid baths at 25 V and 40 V, respectively. The effect of the extended duration of the second anodizing step on pore order degree and structural features of AAO membranes was studied. The presence of alloying elements affects not only the rate of oxide growth but also the microstructure of the anodic film. It was found that pore circularity and regularity of pore arrangement in AAO membranes formed on the AA1050 alloy were always worse than those observed on the pure Al substrate. The structural features, such as pore diameter, interpore distance, wall thickness, barrier layer thickness, porosity and pore density of porous anodic alumina formed on AA1050 are a little different from those obtained for high purity Al. The extended time of the second anodizing step, up to 16 h does not affect significantly the regularity of pore order and all structural features of AAO membranes, independently of the anodizing electrolyte.     

A study on the formation of titania nanopillars during porous anodic alumina through-mask anodization of Ti substrates

The formation of anodic titania during porous anodic alumina (PAA) through-mask anodization has been analysed for varying anodization conditions on mechanically polished bulk Ti surfaces. Titania nanopillars were formed through the porous masks in both oxalic and phosphoric acid electrolytes. For applied potentials above 40 V the titania formed along narrow channels through the alumina pore bottoms resulting in root-like attachments of the titania pillars to the Ti substrate. We further demonstrated that high-field anodization can be used for PAA through-mask anodization. The formation of titania changed with increased current density which resulted in more efficient oxide growth through the alumina pores. When the Al/Ti samples were immersed in the electrolyte without exclusively exposing the Al surface to the electrolyte the titania formed solely on top of the alumina pore bottoms which resulted in that the titania structures were detached from the Ti substrates during selective removal of the PAA templates.     

Ion beam-induced positive imaging of polyimide via two step imidization

The preparation of ion track membranes of thermally stable polyimide films has been performed by ion beam irradiation of partially imidized polyamic acid (PAA) films followed by alkaline etching and final imidization. No discernible positive hole patterns were observed on the irradiated films of partially imidized PAA containing sulfonyl linkages, although sulfonyl group is known to be highly sensitive to ion beams. In contrast, positive hole patterns appeared on the irradiated films of the partially imidized PAA with 68-89% imidization degrees that contains sulfonyl linkages along with a methylene group in the main chain. The most contrasty hole patterns with 0.3 μm diameter were observed on the irradiated PAA films of 89% imidization degree. The irradiated PAA film with the hole patterns was then transformed to the corresponding polyimide film with curing at 350 °C for 1 h. From the structural comparison of the polyimides, the possible mechanism for the hole pattern formation is suggested.     

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.     

Embedded space charge in porous alumina films formed in phosphoric acid

The embedded charge in the barrier layers of porous alumina, formed potentiostatically in phosphoric acid was studied as a function of anodizing voltage (30-57 V) and bath temperature (18 and 21 °C). For that, the polarization measurements of as-grown and annealed alumina/Al samples were conducted in the same anodizing bath by anodic potential sweep at a scan rate of 2.6 V/min. The plane capacitor model was used for the assessment of the charge density in the barrier layers of as-grown porous alumina. For the barrier layers of films formed at 18 °C this value equals to 0.747 μC cm⁻² and does not depend on the anodizing voltage. Increase in electrolyte temperature rises the embedded charge density. Polarization measurements carried out in this paper clearly present that the barriers of phosphoric acid films grown at the anodizing voltages lower than 39 V contain a layer of virtual cathode while at higher voltages this layer disappers. The obtained results allow speaking about promising opportunities of potentiodynamic polarization measurements of alumina films in the same anodizing solution before and after annealing for the studies of charges embedded within the alumina barriers and for the regularities of ion transport.     

The effect of thickness on the composition of passive films on a Ti-50Zr at% alloy

Anodic films were grown potentiodynamically in different electrolytes (pH = 1-14) on a Ti-50Zr at% cast alloy, obtained by fusion in a voltaic arc under argon atmosphere. The thickness of the films was varied by changing formation potential from the open circuit potential up to about 9 V; growth was followed by 30 min stabilization at the forming potential. Films having different thicknesses were characterized by photocurrent spectroscopy (PCS) and electrochemical impedance spectroscopy (EIS). Moreover, film composition was analyzed by X-ray photoelectron spectroscopy (XPS).Regardless of the anodizing conditions, passive films on the Ti-50Zr at% alloy consist of a single layer mixed oxide phase containing both TiO₂ and ZrO₂ groups. However, an enrichment of Ti within the passive film, increasing with the film thickness, is detected both by PCS and XPS. This leads to concentration profiles of Ti⁴⁺ and Zr⁴⁺ ions along the thickness, and to different electronic properties of very thin films (more insulating) with respect to thicker films (more semiconducting), as revealed by the photocurrent-potential curves.     

Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm⁻³ K₂HPO₄ + 0.2 mol dm⁻³ K₃PO₄/glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m⁻². The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.     

Electrodeposition of PbTe thin films from acidic nitrate baths

Electrodeposition of PbTe thin films from an acidic nitric bath was systematically investigated to understand the kinetics and the effect of electrodeposition conditions on film composition, crystallographic structure, texture and grain size. The electroanalytical studies employed initially with a rotating disk electrode to investigate the kinetics associated with Te, Pb and PbTe electrodeposition. The results indicated that the PbTe thin films were obtained by the underpotential deposition (UPD) of Pb atoms onto the overpotentially deposited Te atoms on a substrate.Based on these studies, PbTe thin films were potentiostatically electrodeposited using e-beam evaporated gold thin films on silicon substrate to investigate the effect of various deposition conditions on film composition and microstructure. The data indicated that the microstructure, composition and preferred film growth orientation of PbTe thin films strongly depended on the applied potential and electrolyte concentration. At −0.12 V, the film was granular, dense, and preferentially oriented in the [1 0 0] direction. At potentials more negative than −0.15 V, the film was dendritic and preferentially oriented in the [2 1 1] direction. A smooth, dense and crystalline film with nearly stoichiometric composition was obtained at −0.12 V from a solution containing 0.01 M HTeO₂⁺, 0.05 Pb²⁺ and 1 M HNO₃.     

Influence of pulse plating parameters on the electrocodeposition of matrix metal nanocomposites

Electrocodeposition of alumina particles with copper and nickel from acidic electrolytes has been investigated using different deposition techniques. Compared to direct current (DC) deposition, both pulse plating (PP) and pulse-reverse plating (PRP) facilitated higher amounts of particle incorporation. With conventional DC plating the maximum alumina incorporation is ∼1.5 wt% in a nickel and ∼3.5 wt% in a copper matrix. However, the implementation of rectangular current pulses can give considerably higher particle contents in the metal layer. A maximum incorporation of 5.6 wt% Al₂O₃ in a copper matrix was obtained by PP at a peak current of 10 A dm⁻², a duty cycle of 10% and a pulse frequency of 8 Hz. In general, low duty cycles and high pulse frequencies lead to an enhanced particle codeposition. The microstructure and the hardness of both pure metal films and nanocomposite coatings showed only a weak dependence on the PP and PRP conditions.     

Pore diameter control of anodic aluminum oxide with ordered array of nanopores

Highly uniform, self-ordered anodic aluminum oxide (AAO) with an ordered nanoporous array can be effectively formed from industrially pure (99.5%) aluminum sheets through an anodizing program in a mixture solution of sulfuric and oxalic acids. The influences of anodizing variables, such as applied voltage, solution temperature, oxalic acid concentration, agitation rate, and sulfuric acid concentration, on the average pore diameter of AAO were systematically investigated using fractional factorial design (FFD). The applied voltage, and sulfuric acid concentration were found to be the key factors affecting the pore diameter of AAO films in the FFD study. The pore diameter of AAO is regularly increased from ca. 50 to 150 nm when the applied voltage and the concentration of sulfuric acid are gradually increased from 53 to 80 V and from 3.5 to 8 M, respectively. Fine tuning of the pore diameter for AAO films with an ordered, nanoporous, arrayed structure from industrially pure aluminum sheets can be achieved.     

Growth of multioxide planar film with the nanoscale inner structure via anodizing Al/Ta layers on Si

An Al/Ta bilayer specimen prepared by a successive sputter-deposition of a 150-nm tantalum layer and a 180-nm aluminium layer onto a silicon wafer is anodically processed in a sequence of steps in oxalic acid electrolytes, at voltages of up to 53 V, which generates a 260-nm alumina film with well-ordered nanoporous structure. Further potentiodynamic reanodizing the specimen to 220 V causes the simultaneous growth of a 65-nm tantalum oxide layer beneath the alumina film and an array of oxide ‘nanocolumns’ (∼50 mn wide, ∼80 nm apart, ∼7 × 10⁹ cm⁻² population density) penetrating the alumina pores and reaching precisely to the top of the alumina film. The complete filling of the alumina pores is assisted by the high Pilling-Bedworth ratio for Ta/Ta₂O₅ and a substantially increased transport number for tantalum species (0.4), which is an average value of all migrating tantalum ions with different oxidation states. The nanocolumns are shown to be composed of a unique, regular mixture of Ta₂O₅ (dominating amount), suboxides TaO₂ and TaO_x (0.5 < x < 1), Al₂O₃, metallic Ta and Al aggregates, tantalum diboride (TaB₂) and oxidized boron from the electrolyte. The ionic transport processes determining the self-organized growth of these planar oxide nanostructures are considered and described conceptually.     

Bias voltage effect on the structure and property of the (Ti:Cu)-DLC films fabricated by cathodic arc plasma

Titanium copper-diamond-like carbon (Ti:Cu)-DLC films were deposited onto silicon via cathodic arc evaporation process using titanium (Ti) and copper (Cu) target arc sources to provide Ti and Cu in the metal containing diamond-like carbon (Me-DLC) film systen. Acetylene reactive gases served as the carbon sources, which were activated at 180 °C at 20 mTorr. Varying substrate bias voltages from − 80 to − 250 V were used in order to provide the (Ti:Cu)-DLC structure. The structure, interface, and mechanical properties of the produced film were analyzed by transmission electron microscope (TEM), IR Fourier transform (FTIR) spectra, and Rockwell C hardness. The process parameters were compared by studying the various mechanical properties of the films, such as microhardness and adhesion. The results showed that the Ti-containing a-C:H/Cu coatings exhibited an amorphous layer of Ti-DLC layer as well as a nanocrystalline layer of Cu multilayer structure. The mechanical properties of the coatings, as determined by Rockwell C indents testing and the ball-on-disc test, varied with the the applied negative DC bias voltage. These (Ti:Cu)-DLC coatings are promising materials for soft substrate protective coatings.     

Effect of aluminum annealing on the galvanoluminescence properties of anodic oxide films formed in organic electrolytes

The galvanoluminescence (GL) properties of anodic oxide films formed in organic electrolytes were investigated at different aluminum annealing temperatures. The results of the spectral measurements showed two different types of GL sources: carboxylate ions incorporated in oxide films during the anodization and the molecules AlH, AlO, Al₂, AlH₂, also formed during anodization process and already recognized in the case of inorganic electrolytes. The latter was related to gamma alumina crystalline regions formed by annealing of the aluminum samples at temperatures above 500 °C.     

Flow modulated ionic migration during porous oxide growth on aluminium

The ability to tailor the morphology of porous anodic alumina (PAA) drives applications in the important architectural, aerospace, packaging, lithography and nanotechnology sectors with the mechanisms regulating oxide growth being pursued vigorously. Here, we have probed the barrier layer of PAA during film growth, using high voltage, in situ, electrochemical impedance spectroscopy. Our new findings account for aluminium oxidation at the metal/oxide interface, ionic migration and interaction of charge carriers through the layer, and oxide viscous flow under the field. Separation of these processes by appropriate modelling discloses that ionic migration, being driven by the electrical field only, is independent of the electrolyte. The incorporated electrolyte-derived species modify the mechanical properties of the oxide, by regulating the oxide displacement from the barrier layer. The approach developed provides the vital foundations for an integrated theory of porous film growth on a range of metals in various film-forming electrolytes.     

Remote electro-precipitation of transparent ZnO on nano-porous alumina template

Zinc oxide (ZnO) thin film was deposited onto non-conducting alumina template by remote electrochemical and precipitation method. Chronoamperometry (i.e., constant potential mode) of −0.75 V (versus Ag/AgCl) was applied to deposit compact ZnO thin film onto the honeycomb shaped pores of a nano-porous alumina membrane. Analyzing morphological observations by SEM, we observed three distinct regions: (i) conducting platinum substrate; (ii) non-conducting alumina interlayer; (iii) thin film onto alumina template. XRD structural analysis of deposited materials onto alumina template and EDX analysis of alumina layer clearly indicated the formation of ZnO thin film was formed onto non-conducting alumina template not inside of alumina template. This remote-deposited ZnO showed more compact ZnO-hexagonal structure and a little bit higher transmittance than conventionally prepared ZnO onto ITO glass. Experimental observations were further discussed via the understanding of the mechanistic origin of ZnO formation onto alumina template.     

Self-standing polyaniline film of mono-particle layer

Large, uniform and stable polyaniline (PANI) films were fabricated by ordering and drying polyaniline-coated-polystyrene (PANI-PS) latex microspheres. Particles on a glass substrate were highly ordered, closely packed and formed a two-dimensional lattice structure with the thickness of the diameter of the particle, 3.2 μm. Films fabricated on the substrate were peeled off by the pressure of water to yield the floating film, which could be transferred onto other substrates. One end of the film was connected to the platinum plate electrode while the other end was put in sulfuric acid. The film to which voltage was applied showed color change corresponding to the redox reaction. The voltammogram of the film showed the cathodic and the anodic peaks at 0.20 and 0.35 V versus SCE, respectively, being similar to peaks of electropolymerized PANI films. Films were inserted into tetrahydrofuran to remove the core-polystyrene. The resulting film had similar properties as polyaniline films of PANI-PS latex microspheres.     

Anodic formation of low-aspect-ratio porous alumina films for metal-oxide sensor application

Thin nanoporous anodic alumina films, of low aspect ratio (1:1), with two distinctive pore sizes and morphologies were prepared by two-step constant-current anodising of aluminium layers on SiO₂/Si substrates in 0.4 mol dm⁻³ tartaric (TA) and malonic acid (MA) electrolytes and then modified by open-circuit dissolution. The anodic films were employed as a support material for sputtering-deposition of thin WO₃ layers in view of exploiting their gas sensing properties. The films and deposits were characterized by scanning electron microscopy, X-ray diffraction and electric resistance measurements at fixed temperatures in the range of 100-300 °C upon NH₃ and CO gas exposures. Test sensors prepared from the annealed and stabilized alumina-supported WO₃ active layers were insensitive to CO but showed considerably enhanced responses to NH₃ at 300 °C, the sensitivity depending upon the anodic film nature, the pore size and the surface morphology. The increased sensor sensitivity is due to the substantially enlarged film surface area of the TA-supported WO₃ films and the nanostructured, camomile-like morphology of the MA-supported WO₃ films. Sensing mechanisms in the alumina-supported WO₃ active layers are discussed.     

多孔膜基材对等离子体接枝膜微观结构的影响

采用等离子体诱导接枝聚合法,在有机聚偏氟乙烯(PVDF)膜、尼龙(N6)膜和无机多孔阳极氧化铝(PAA)膜上接枝甲基丙烯酸(MAA),系统地研究了多孔膜基材及接枝工艺条件对接枝开关膜微观结构的影响。结果发现,对于PVDF基材膜,单体溶液脱气次数越多接枝率越大。对于不同类别的多孔膜基材,采用相同工艺条件接枝时,N6膜的接枝率高于PVDF膜,PAA膜接枝率最低。在接枝率相近的时候,接枝层在PVDF膜整个断面孔内和表面均存在,而N6膜则主要集中在表层。在PAA膜上接枝时发现,等离子体照射功率的强弱对基材膜的影响很大,当功率超过50W时,PAA膜表层就会被刻蚀而导致膜的质量减轻。     

中性介质铝表面无机非金属膜层的电化学沉积 (Ⅲ)无机非金属膜层成膜机理初探

通过分别测定铝在成膜促进剂、络合成膜剂和Na2WO4的不同组合体系中的稳态伏安特性曲线和暂态阳极氧化曲线,及铝在成膜促进剂＋络合成膜剂＋Na2WO4体系中不同氧化阶段形成的膜层的显微形貌,初步提出了无机非金属膜层的成膜机理,即在特殊电解液和工艺条件下,铝／电解液界面上形成带负电含铝胶体粒子,由于界面化学、电化学和焦尔热的作用,胶体层在阳极不均匀沉积、浓缩、脱水和快速冷却,导致具有特殊结构和性能的无机非金属膜的形成。